Jeff Longtin

Welcome to the Forum Carol.
Do we have any idea what the mask is made of?
Short answer to your question: more than likely some damage MAY occur. (Especially if you have no idea what the antique is made of.)
Alginate is probably the least harmful but it is water based so the piece may see some water damage. Urethane rubber removes the water element but the piece would need to be sealed with a silicone like substance which may affect the piece.
A photo would also help.

Min has picked up on what I was suggesting. 
If you fire a cone form on TOP of a cone form you will create a stressful situation, for the clay, as it shrinks in the firing. (The clay will shrink around the cone and probably crack as a result.)
If you fire a cone shape with the cone pointing downward, in a setter that is also a downward cone, a bowl basically, then the cone will only sink deeper into the cone as it shrinks. As there is nothing inside the cone you are not putting stress on the clay. The likely hood is greater that it will retain its cone shape this way.
Leaving the middle of the cone support open allows the clay to shrink and sink without interference. If there is no hole then you run the risk of the cone bottoming out in the middle of the setter.

Min has picked up on what I was suggesting. 
If you fire a cone form on TOP of a cone form you will create a stressful situation, for the clay, as it shrinks in the firing. (The clay will shrink around the cone and probably crack as a result.)
If you fire a cone shape with the cone pointing downward, in a setter that is also a downward cone, a bowl basically, then the cone will only sink deeper into the cone as it shrinks. As there is nothing inside the cone you are not putting stress on the clay. The likely hood is greater that it will retain its cone shape this way.
Leaving the middle of the cone support open allows the clay to shrink and sink without interference. If there is no hole then you run the risk of the cone bottoming out in the middle of the setter.

It will work if the cup was fired to maturity first and then a lower fire glaze applied and fired without the setter.
Even though there is very little shrinkage from greenware to bisque fired I would hazard a guess there is enough to crack this piece where the little nub in the middle is.   As the clay shrinks it will squeeze up against the nub, nub won't shrink, clay will and it will crack. 
Why are you only worried about the bisque of this form? Will you leave it unglazed or ?
If you have access to spray glazing the finished pieces I would use your setter design as above but flip the slack over and have the clay fit the inside of the setter, including the nub then fire to maturity first then do a low fire spray glaze and re-fire.

Have you thought of firing the piece upside down? (for the bisque anyway) Build up a mound of sand, as Callie suggests, and then push the upside down cone shape into it. It won't help you in the glaze firing but at least you can reduce distortion in the bisque.

Separate the two processes: 1) filling the seams/gaps, and 2) filling the mold.
1) pour a small amount of slip into the mold and use a brush to push the slip into the seams/gaps. Wait a minute or two. Use the brush a second time to push the slip into the seam/gaps. Wait another minute or two. Do the same thing a third time if necessary. Wait another minute or two.
After you see the seam/gaps sufficiently filled THEN go ahead and start filling the mold. Go slowly to make sure your gaps are filled and no slip is leaking out. 
If it looks like you filled the gaps sit back and breathe a sigh of relief.
Right now my purse vase molds are just plaster parts held together with straps and rubber bands. The shape, 8" wide, 8" tall, 3" deep, is really susceptible to blowout if I forget to seal the seams. The above method assures that the slips stays in place and I get a decent casting without a lot of mess or stress.
 
 

Separate the two processes: 1) filling the seams/gaps, and 2) filling the mold.
1) pour a small amount of slip into the mold and use a brush to push the slip into the seams/gaps. Wait a minute or two. Use the brush a second time to push the slip into the seam/gaps. Wait another minute or two. Do the same thing a third time if necessary. Wait another minute or two.
After you see the seam/gaps sufficiently filled THEN go ahead and start filling the mold. Go slowly to make sure your gaps are filled and no slip is leaking out. 
If it looks like you filled the gaps sit back and breathe a sigh of relief.
Right now my purse vase molds are just plaster parts held together with straps and rubber bands. The shape, 8" wide, 8" tall, 3" deep, is really susceptible to blowout if I forget to seal the seams. The above method assures that the slips stays in place and I get a decent casting without a lot of mess or stress.
 
 

As best as I can tell here's what's happening in this situation...
when the liquid slip first makes contact with the dry plaster surface it IMMEDIATELY dries. Then, as more slip continues to flow over it, it forms a "ridge". 
In theory you have two solutions: as I mentioned previously, you can make the point of contact an area not visible from the top OR you can reduce the degree, to which the plaster dries the slip, at that first point of contact. You would do this by wetting the mold before you start casting. (Simply hold the mold, under running water, for just a second.)
Wetting the mold may reduce the height of the bump but it will also slow the casting process. Therein you have a choice.
Porcelain doll artists first introduced me to the idea of wetting a mold before casting. (To reduce sticking.) Subsequently I found it has other benefits as well.

When I make a plaster form it usually takes 5-7 days before its completely dry. I place plaster forms on plastic grates that allow air to freely flow around the object. If I forget to prop the piece properly and leave it on a wood board, or plastic sheet, it will develop a brown/green mold where it was in contact with the wood or plastic.
"Brushing off" - unfortunately this is subjective. When you pick up a plaster mold/form you will usually have some white powder on your hands/fingers. An indication that the plaster form is deteriorating would include an increasing roughness to the plaster surface.
 

Quick Google search turns up Glacial Silt is also called Glacial Flour or Rock Flour which is made from tiny clay sized particles of very finely ground particles of rock.  Particles of the Glacial Silt/flour can be so small that will suspend in water, this is what makes some glacial lakes look such a beautiful shade of turquoise. Makes sense that it is loaded with silica. Nothing at all like rock salt which is well, salt. 

I assume they’re using glacier grit because it’s locally available, but surely there must be a larger mesh size available. I dunno. 
@Jeff Longtin road salt used by itself is pretty caustic, and presents issues for roadside plants, as well as issues of it getting into waterways. Lots of municipalities will mix grit with salt to reduce the amount of salt used and increase traction. The mix will vary with climate and whatever gravel source is available locally. The stuff my city uses is like pea gravel, and it’s nowhere near that dusty. That said, our municipality does do as Rae mentioned, and they send out water trucks and street sweepers in the spring. Partly to keep the dust down, partly to keep it out of the storm drains, and partly to reclaim some of the road sand to reuse the next year.  
 

Having worked in a pottery shop, where lots of young people are convinced that one day in a pottery shop exposes one to a lifetime of silicosis, I'll offer this thought:
Did you see the interview with the "industrial hygienist"? Did you see how cluttered his office is? Do you think a cluttered office is cleaned regularly?
I think the hygienist might want to clean his office before he's interviewed on tv again. 
That said, I had no idea "glacier silt" is actually a thing. (I presume its a form of salt?) Makes me wonder if "road salt" used all over the US is also creating a similar problem? 
I appreciate the posting.

Sometimes I combine mold parts to test out new design ideas.  Sometimes the mold parts do not fit well together. I find filling the small holes/gaps with slip, FROM THE OUTSIDE, to be  a simple remedy. Let the slip set up for a few minutes and you have a "sealed" mold.  THEN fill the mold cavity with slip.
You can also fill the gaps from the inside but I find it hard to get my hands inside smaller molds.
 
 

The key to Murphy's is to create a lather. (with a brush) Then wiping away the excess lather with a sponge, then wiping away excess bubbles with a dry brush. Do this three times and you'll have a nice "oil-like" surface. (Which flakes off/dissolves in a few days.)
My concern with pouring plaster against wet Murphy's is that the wet substance MAY absorb into the plaster surface. You might have a different experience, however?
Another option is to use a food safe silicone spray. In the US there is a product called PAM. In another mold making forum this was mentioned as an effective release material when pouring plaster onto nonporous surfaces.

Having worked in a pottery shop, where lots of young people are convinced that one day in a pottery shop exposes one to a lifetime of silicosis, I'll offer this thought:
Did you see the interview with the "industrial hygienist"? Did you see how cluttered his office is? Do you think a cluttered office is cleaned regularly?
I think the hygienist might want to clean his office before he's interviewed on tv again. 
That said, I had no idea "glacier silt" is actually a thing. (I presume its a form of salt?) Makes me wonder if "road salt" used all over the US is also creating a similar problem? 
I appreciate the posting.

I've not used blue spark but I have used Palladium quite a bit. 
I apply 5 coats. (to a 06 bisque pot, porcelain) The first two coats are thin. The remaining three coats are progressively thicker.  I only use bottle thick as the final coat. 
I'm not used to brushing glazes so the bottle thick always scares me. (Along the lines of what Dick mentions.) Bottle thick always seems to leave brush strokes/pinholes so I find the two thin layers prevents that from happening.

Hello Sue. Either side will dry the clay equally well. The smooth side is simply easier to clean so I would use that side.
The next time you pour you might take a moment to watch the plaster set. At the 10 - 15 minute point the plaster will lose its water surface and start hardening. If you catch it in time you can use a straight edge scraper and lightly scrap the surface of the plaster. If you do this several times you will slowly develop a smooth surface. 
Its an acquired skill but one I use alot so that my textured plaster forms can easy sit on level surfaces. (Which reduces that chance they might warp as they dry.) 

Hello Sue. Either side will dry the clay equally well. The smooth side is simply easier to clean so I would use that side.
The next time you pour you might take a moment to watch the plaster set. At the 10 - 15 minute point the plaster will lose its water surface and start hardening. If you catch it in time you can use a straight edge scraper and lightly scrap the surface of the plaster. If you do this several times you will slowly develop a smooth surface. 
Its an acquired skill but one I use alot so that my textured plaster forms can easy sit on level surfaces. (Which reduces that chance they might warp as they dry.) 

Hello Sue. Either side will dry the clay equally well. The smooth side is simply easier to clean so I would use that side.
The next time you pour you might take a moment to watch the plaster set. At the 10 - 15 minute point the plaster will lose its water surface and start hardening. If you catch it in time you can use a straight edge scraper and lightly scrap the surface of the plaster. If you do this several times you will slowly develop a smooth surface. 
Its an acquired skill but one I use alot so that my textured plaster forms can easy sit on level surfaces. (Which reduces that chance they might warp as they dry.) 

I'll put in a second for the Fred Olsen Kiln Book. His little Fast Fire kiln is easy to build and easy to fire. Plus, it's not too big, 10 cu ft. I helped build one years ago at the MN Ren Festival. Two fire boxes are a little tedious but it made it super easy to control.  Usually got to cone 7 just as the Festival was closing. (Once people were gone we'd throw in salt.) 
 

Mark makes a good point. Glue is for broken molds, two pieces, not "cracked".
Cracks, on the inside of the mold, can sometimes be filled with a thin slurry of plaster, but sometimes not. It's usually best to apply the plaster in a water like consistency and build it up in layers. This repaired crack will likely be a spot of sticking, for the cast piece, but after several castings the sticking should decrease.
 
 

The glue that is a step above Elmers is Titebond wood glue. Its like Elmers in all reguards except its waterproff when dry

Heating a mold - so you know, 120 degrees is the highest temperature you should expose plaster too. (Beyond that it starts to break down.) 
The cracks could indicate a problem that simply gets worse, and a new form needs to be made, or you may get lucky, and they don't increase?
I find Elmers glue to work well to temporarily repair cracked molds. (Elmer's is a simple, white, water soluble glue, widely available in the US.) However, once the plaster becomes sufficiently wet, the glue sometimes loses its bonding ability and the pieces separate again.

Hello Henrietta,
Welcome to the Forum. 
My suggestion is that you should not wait 5 minutes for the plaster to "get wet". Two minutes is all I leave it for. (Both USG and Georgia Pacific recommend the same.) You really just want the particles to get moistened. 
The key to plaster strength is the mixing. Shorten the wet time from 5 minutes to 2 and make up the difference in mixing.  That way you can pour while the mix is still fluid but you also have plenty of mix time.
Good luck!

Murphy's Oil Soap is an effective way to seal plaster, temporarily. Depending upon how it is applied it will sometimes just flake off the plaster surface after the plaster has dried. (No need for washing, in other words.)
Murphy's, like all soaps, develops a lattice structure only after it is lathered up and then allowed to dry. (After the suds/bubbles have been wiped away.) The drying creates the lattice structure that seals the surface.
If one is making a mold of a fired ceramic object, Murphy's doesn't really seal the surface. It sits on top. If the plaster is poured while the Murphy's is still wet it may absorb into the freshly poured surface.  If that happens the plaster may become weakened and may erode sooner than normal.
If I'm making a mold of something, other than plaster, and I'm unsure if it's completely nonporous, I will spray acrylic paint on the object. (That way I know the surface is sealed.)
In mold making, like all things creative, there are "rules" that can make the process more effective. On the other hand, like all things creative, one can break the rules and still have successful outcomes.